1. Field of the Invention
The present invention relates to an audio system adapted to record or transmit four or less individual channels of audio information containing directional information on a two-track recording medium or transmitting medium and to reproduce the recorded or transmitted information as four or less discrete audio output signals.
2. Description of the Prior Art
Audio recording and reproducing systems sometimes called stereo-quadraphonic because they involve more channels than a stereophonic system, are generally divided into matrix types and carrier types at present.
By way of example, the type referred to as the SQ system is a matrix system. It includes left front, left back, right back and right front signals L.sub.F, L.sub.B, R.sub.B, and R.sub.F encoded into two composite signals L.sub.T and R.sub.T according to the following equation: EQU L.sub.T = L.sub.F + 0.707 R.sub.B - j0.707 L.sub.B EQU r.sub.t = r.sub.f - 0.707 l.sub.b = j0.707 R.sub.B ( 1)
the composite signals L.sub.T and R.sub.T expressed by equation (1) are decoded by a decoder to reproduce the original signals according to the following equation: EQU L.sub.F ' = L.sub.F + 0.707 R.sub.B - j0.707 L.sub.B EQU L.sub.B ' = L.sub.B - 0.070 R.sub.F + j0.707 L.sub.F EQU R.sub.B ' = R.sub.B + 0.707 L.sub.F - j0.707 R.sub.F EQU R.sub.F ' = R.sub.F - 0.707 L.sub.B + j0.707 R.sub.B ( 2)
since four original signals are thus converted into two composite signals by a matrix, the SQ system is compatible with prior art stereophonic record players and hence two-channel stereophonic reproducing apparatus can, without change, reproduce the two composite signals. The four reproduced signals expressed by equations (2) correspond to the four original signals, but other signals are mixed with them as crosstalk signals. As a result, the separation of the four signals cannot be complete. For this reason, decoders using a special logic circuit have been proposed to obtain a degree of separation that is sufficient from a practical point of view. However, no separation equal to that of a discrete type is possible at present.
A carrier type of the quadraphonic system, called a CD-4 system, is disclosed in U.S. Pat. No. 3,686,471. According to that system, a first composite signal consisting of a main channel signal L.sub.F + L.sub.B and a sub-channel signal which is obtained by angularly modulating a carrier signal of 30 KHz with a subtraction signal L.sub.F - L.sub.B, and a second composite signal consisting of a main channel R.sub.F + R.sub.B and a sub-channel signal, which is obtained by angularly modulating the carrier signal of 30 KHz with a subtraction signal R.sub.F - R.sub.B, are recorded on one sound groove. Four discrete signals L.sub.F, L.sub.B, R.sub.B and R.sub.F are obtained at the reproducing side by demodulation and matrixing. In this case, the band of the sub-channel signal is selected, for example, between 20 KHZ and 50 KHz, so that a specially designed pickup cartridge is required, which is difficult to obtain.
U.S. Pat. No. 3,761,628 discloses a method by which four discrete signals are reproduced by a conventional pickup cartridge. With this system, a matrixed signal is contained in a main channel signal and a modulated signal is contained in a sub-channel signal. The modulated signal contains a signal component which may cancel the crosstalk signals of the main channel at the reproducing stage. However, since signals contained in the sub-channel are in the form of a single side band (SSB) signal, it is not possible to transmit or reproduce information with a good signal-to-noise (S/N) ratio.